Cardiovascular disease accounts for nearly 40% of all deaths that occur in the United States each year, and the cost to the US economy, $394 billion (American Heart Association; 2005), is almost 14 times greater than the NIH total annual budget (FY 2007 $28.4 billion). With respect to that scope, small improvements in therapeutics may reflect enormous benefits. The overall goal of this application is to control recombinant Adeno-associated virus (rAAV) therapeutic transgene expression in the heart after systemic delivery. To examine expression in failing hearts we will employ a standard model of heart disease. The LIM knockouts will be used as our model of hypertrophy. Two methods will be employed to restrict rAAV mediated transgene expression to the heart: 1) Comparisons between the best parental and transcapsidation (combination of capsid subunits from different AAV serotypes into one virion) serotypes, will permit examination of cardiac tropism of the recombinant virus after systemic injection; and 2) Heart specific enhancer/promoter configurations that confer gene expression induced by heart failure will be used to further aid in the specificity of rAAV delivery to the target tissue. Both AAV serotypes 6 and 9 demonstrate significant transgene expression in the heart after systemic tail vein and direct intra-coronary delivery. Quantification of luciferase expression and viral genome copy (vg) number in the heart after delivery of equivalent numbers of viral particles demonstrates that AAV9 delivery results in two-fold greater vg after tail vein injection than AAV6, while AAV6 has greater vg after intra-coronary delivery. AAV6 has a more restricted natural tropism for cardiac tissue, with up to 15% of total light units in the thoracic cavity compared with less than 2% for AAV9. Understanding the amino acids that govern AAV6s' natural tropism for the heart is one objective of this application. A second objective is to combine properties through viral transcapsidation to synergize advantageous phenotypes into one virion. Taking advantage of the cardiac tropism of AAV6 and robust expression from AAV9 as well as the best transcapsidation mixtures we will assess the therapeutic function of 2ARKct and S100A1 transgenes, after systemic injection, in a dose dependent manner. Finally, with the goal of further limiting cardiac gene expression, we have designed enhancer/promoter configurations to bias transgene expression to cardiac tissue. NARRATIVE In the United States heart failure is a major cause of mortality. In addition, the cost of hospitalization and missed work is a drain on our economy. As the molecular changes that underpin the disease are elucidated genetic treatments are beginning to show benefit in animal models. The expression of several genes in animal models of Heart Failure improves the hearts ability to pump without ventricular dilation. However, transgenic and knockout models cannot be easily adapted to human heart failure, therefore, mechanism for gene delivery to the heart become more relevant to these future therapeutic efforts. An end point for this application is to determine the therapeutic dose of a therapeutic gene delivered by an experimental gene transfer vector.